1. Field of the Invention
The present invention relates to an optical scanning apparatus used in an image forming apparatus, such as a laser beam printer. More particularly, the present invention relates to an optical scanning apparatus configured to deflect and scan a laser beam by using a deflecting mirror (hereinafter referred to as an “MEMS mirror”) that is manufactured on the basis of the technique called MEMS (Micro Electro Mechanical System).
2. Description of the Related Art
Japanese Patent Laid-Open No. 2005-195869 proposes an optical deflection device using a MEMS mirror that is operated to reciprocate with resonance oscillations. In comparison with an optical deflection device using a rotatable multi-faceted mirror such as a polygonal mirror, the optical deflection device using the MEMS mirror has the following superior features. The size of the optical deflection device can be greatly reduced. Power consumption is small. Theoretically, a mirror surface causes no face tangle. In particular, when the optical deflection device employs a MEMS mirror that is manufactured by using a single-crystal Si (silicon) through a semiconductor process, the optical deflection device is theoretically free from metal fatigue and has superior durability.
FIG. 14 illustrates the construction of an optical scanning apparatus proposed in Japanese Patent Laid-Open No. 2005-195869.
A laser beam emitted from a laser beam source 162 is introduced to a MEMS mirror 651 through a collimator lens 631 and a cylindrical lens 632. The MEMS mirror 651 is operated to reciprocate by a driving unit (not shown) so that the laser beam is deflected and scanned.
Most part of the deflected and scanned laser beam forms, through a scanning lens 166 and a folding mirror 168, an image on a scanned surface and is used for writing the image.
Parts of the deflected and scanned laser beam are changed in their directions by upstream-of-sensor mirrors 169a and 169b which are disposed respectively upstream of light receiving sensors 160a and 160b, and those beam parts enter the light receiving sensors 160a and 160b. The light receiving sensors 160a and 160b detect the entered laser beam electric signals and output electric signals. Reference numeral 161 denotes an optical box in which the aforementioned various components are mounted. Reference numeral 165 denotes a MEMS mirror holder for holding the MEMS mirror 651.
On the basis of the electric signals output from the light receiving sensors 160a and 160b, control is executed by monitoring, e.g., a deviation between the resonance frequency of the MEMS mirror 651 and the driving frequency of the driving unit (not shown), and changing the driving frequency and the amplitude of the driving unit (not shown) so as to provide the desired deflecting operation.
In the above-described related art, however, when the driving of the MEMS mirror 651 is controlled on the basis of the outputs of the light receiving sensors 160a and 160b, the laser beam is deflected by the reciprocating operation of the MEMS mirror 651. Therefore, the deflected and scanned laser beam enters twice each of the light receiving sensors 160a and 160b per cycle of the reciprocating operation of the MEMS mirror 651. In that case, the two laser beams entering each of the light receiving sensors are scanned bidirectionally (in opposed directions). When an electric signal at the timing at which the laser beam enters the light receiving sensor is used as a control signal for each of the opposed scanning directions, the laser beams scanned in the opposed directions differ in position and incident angle at which those laser beams start to enter a light receiving surface of the light receiving sensor, because the light receiving surface of the light receiving sensor generally has a certain width. In other words, the method of detecting the timing at which the laser beam starts to enter the light receiving surface has the problems that the laser beam is detected at different deflection angles of the MEMS mirror, and that detection and control cannot be performed in a precise manner.
A solution for overcoming those problems is to use a 2-division sensor. The 2-division sensor includes two light receiving sensors which are disposed close to each other in the scanning direction and which successively issue outputs upon passage of the laser beam over the respective sensors. The 2-division sensor outputs a signal at the time when the outputs of the two light receiving sensors intersect each other. Accordingly, regardless of whether the laser beam is scanned rightward or leftward with respect to the light receiving surface, the laser beam can be detected in a state deflected at the same deflection angle. However, the 2-division sensor is more expensive than an ordinary light receiving sensor.